Human remains dating from the 1st Century AD suggest tuberculosis (TB) may have killed off leprosy in Europe.
There are still many cases of leprosy around the world
Scientists at University College London have been examining a shrouded body recently discovered in a sealed chamber in Israel.
The bones reveal the man was infected with both TB and leprosy.
Given that TB is the more aggressive and faster-killing of the two, the scientists say it would have won the battle of the diseases.
In the Middle Ages, leprosy was widespread.
Around this time, TB began to spread across Europe and overtook leprosy, which has become a relatively rare disease now in comparison to TB.
There is a theory that having one of these diseases protects a person against contracting the other one, which is called cross immunity.
Some say this theory explains the rise of TB and fall of leprosy - more people caught TB and were therefore protected against leprosy.
But Dr Helen Donoghue and Dr Mark Spigelman from UCL's Centre for Infectious Diseases and International health say their findings disprove this notion.
"The fact that you can get active disease with both organisms - TB and leprosy - at once seems to contradict the cross immunity theory," explained Dr Donoghue.
"We believe if a person had both leprosy and TB they did not have time to die from the leprosy and died of TB instead.
"Eventually, that is why leprosy declined in Western Europe," she said.
The first remains the researchers examined, which prompted them to go on and test 32 in total from all over Europe dating from the 1st to the 15th centuries, were discovered by Israeli archaeologist Dr Shimon Gibson.
The body belonged to a man who had lived around the time of Christ in a cave just outside Jerusalem.
"The body was in a rock-carved niche which had been beautifully carved over and covered with a stone so for 2,000 years it remained undisturbed," said Dr Spigelman.
He said there was something special about this body that set it apart from the other 30 or so buried in the same cave.
"The normal burial practice was to wrap it in a shroud, then return after a suitable time and rebury the bones in an ossuary - a plaster casket.
"Our shrouded man had not been reburied."
He said this was odd because the man must have come from a wealthy, important family to have been buried in a place that was the equivalent of London's Westminster Abbey or St Paul's Cathedral.
"It takes a heck of a lot not to uphold religious rituals, which started me thinking perhaps the family were scared to go in to recover the bones.
"That's when leprosy came to mind.
"Mr Shroud had both leprosy and TB."
He said leprosy was a feared disease and that people with leprosy were often cast out of society and shunned.
He said these social factors, combined with the effect of the disease on the body, might weaken the person's immune system.
This, in turn, would make them susceptible to contracting TB or having a reactivation of past TB infection that was lying dormant in the body, he said.
The research findings appear in the latest edition of the Royal Society Proceedings B.
Dr Dianna Lockwood, a leprologist at the London School of Hygiene and Tropical Medicine, said the findings were interesting and that certainly a person with both TB and leprosy was more likely to die from the TB.
However, she said the new theory did not explain why places like India and Ethiopia still have relatively high levels of both leprosy and TB.